Let us now consider a circuit made out of two pieces of metal like lead, oxided across their contact surface (weak link; see e.g. Figs 1.1 and 1.5 ref. [1]}), closed by a conducting cable in which a (dc) battery has been inserted. Immerse the circuit in an appropriate fashion in a container filled with liquid helium (see Fig. 3.9 of ref [1]}). Approach a compass to the system. The needle, which before immersion had reacted to the current induced by the potential difference \(V\)  created by the (dc) battery, with a shift of the needle from the unperturbed South (S)\(\rightarrow \) North (N) (S\(\rightarrow \) N) position to a new one, now indicates the S\(\rightarrow \) N direction, implying that the direct current induced at room temperature by the battery is not circulating anymore. If one had access to something similar to a (non standard)“radio” receiver switched on theElectromagnetic high frequency/far infrared. EHF/FIR region of the electromagnetic spectrum (around the THz region), and tuned it very delicately in search for emitters, one will find a rather weak “station”. It “broadcasts” in a single microwave frequency, implying that along the circuit, an alternating current is circulating –so called (ac) Josephson effect [3]}, [4]}– and consequently, the weak link is emitting photons of that frequency (\(\nu _J=2e\times V/h\) , see e.g. ref. [5]}).
